The Zaydis are the followers of Zayd al-Shahid, the son of Imam al-Sajjad. Zayd rebelled in 121/737 against the Umayyad caliph Hisham 'Abd al-Malik and a group paid allegiance to him. A battle ensued in Kufa between Zayd and the army of the caliph in which Zayd was killed.77)
The followers of Zayd regard him as the fifth Imam of the Household of the Prophet. After him his son, Yahya ibn Zayd, who rebelled against the caliph Walid ibn Yazid and was also killed, took his place. After Yahya, Muhammad ibn 'Abdallah and Ibrahim ibn 'Abdallah, who revolted against the Abbasid caliph Mansur al-Dawaniqi and were also killed, were chosen as Imams.
Henceforth for some time there was disorder in Zaydi ranks until Nasir al-Utrush, a descendant of the brother of Zayd, arose in Khurasan. Being pursued by the governmental authorities in that region, he fled to Mazandaran, becoming himself Imam. For some time his descendants continued to rule as Imams in that area.
According to Zaydi belief any descendant of Fatimah (the daughter of the Prophet) who begins an uprising in the name of defending the truth may become Imam if he is learned in the religious sciences, ethically pure, courageous and generous. Yet for some time after Utrush and his descendants there was no Imam who could bring about an insurrection with the sword until recently when, about sixty years ago, Imam Yahya revolted in the Yemen, which had been part of the Ottoman Empire, made it independent, and began to rule there as Imam. His descendants continued to rule in that region as Imams until very recently.
At the beginning the Zaydis, like Zayd himself, considered the first two caliphs, Abu Bakr and Umar, as their Imams. But after a while some of them began to delete the name of the first two caliphs from the list of Imams and placed Ali as the first Imam.
From what is known of Zaydi beliefs it can be said that in the principles of Islam (usul) they follow a path close to that of the Mu'tazilites, while in the branches or derivative institutions of the law (furu') they apply the jurisprudence of Abu Hanifah, the founder of one of the four Sunni schools of law. They also differ among themselves concerning certain problems.